Organic semiconductors are used in the production of simple electronic components e.g. resistors, diodes, field effect transistors, and also optoelectronic components like organic light emitting devices, e.g. OLED (organic light emitting diodes), and many others. The industrial and economical significance of the organic semiconductors and their devices is reflected in the increased number of devices using organic semiconducting active layers and the increasing industry focus on the subject.
Organic semiconductor devices are made by layers; such organic semiconducting layers mainly comprise conjugated organic compounds, which can be small molecules, for instance monomers, or oligomers, polymers, copolymers, copolymers of conjugated and non-conjugated blocks, completely or partially cross-linked layers, aggregate structures, or brush like structures. A device made with different types of compounds, in different layers or mixed together, for example with polymer and small molecule layers, is also called a polymer—small molecule hybrid device. OLEDs are preferentially made of small molecules because the deposition techniques involved in fabricating small molecule OLEDs enable the fabrication of multilayer structures.
Since 1987, large efforts were undertaken worldwide by research groups and by industrial organizations to improve the performance of OLEDs, especially of small molecule OLEDs. One of the first quests was to find suitable organic semiconductor materials made of small molecules which are able to form homogeneous layers. Nowadays, charge carrier transporting materials for industrial use are morphologically stable at least to the temperature of 85° C., typical materials have glass transition temperatures above 100° C. At the same time the materials need to fulfill a set of other requirements, such as high transparency in the visible spectrum and good charge transporting abilities.
Most of the good performance electron or hole transport materials are relatively high cost materials mainly due to their complex synthetic route, which presents a problem to be solved.
Another problem to be solved is the enhancement of the outcoupling efficiency of OLEDs to be used in lighting. Typical organic light-emitting diodes have the disadvantage that only about 25% of the light produced is emitted from the device. About 50% of the light remains as internal modes in the arrangement of organic layers located between the reflecting electrode and the semitransparent electrode. A further 20% is lost due to total reflection in the substrate. The reason for this is that the light inside an OLED is formed in optical media having a refractive index of about 1.6 to 1.8. If this light now impinges upon an optical medium having a lower refractive index, for example, another layer inside an OLED stack, the substrate on which the OLED is formed or one of the electrodes, total reflection occurs if a certain value of the angle of incidence is exceeded. For improving outcoupling, several different techniques are used, for instance, micro lenses array, as described, e.g. in document US 2010/0224313 A1. However such techniques require further development because light extraction efficiency is still far from 100%.
For the use of OLEDs in the lighting and display field, it is therefore necessary to use suitable outcoupling methods which could furthermore be incorporated inexpensively in the fabrication process. It is assumed that, for lighting applications, an OLED area of 1 cm2 must only cost a few cents in order for its application to be economically reasonable. This means, however, that only particularly inexpensive methods come into consideration at all for increasing the light outcoupling. OLEDs based on so-called small molecules (SM) are nowadays processed with, i.a., the aid of thermal evaporation in vacuum. Typically, OLEDs consist of two to twenty layers which are all individually thermally vapour-deposited. If the outcoupling can now be significantly improved by means of just one more single thermally vapour-deposited layer, the condition on the costs of the outcoupling method will be satisfied in any case. The same applies to SM-polymer hybrid OLEDs.
For applications of OLEDs as lighting elements, it is furthermore necessary to make the devices having a large-area. For instance, if an OLED is operated at a brightness of 1000 cd/m2, areas in the range of a few square meters are required to illuminate an office space.